1. Field of the Invention
The present invention relates to a composition for film formation, which can be formed into a porous film that excels in dielectric properties, adhesion, film consistency and mechanical strength, and has reduced absorption; a porous film and a method for forming the same; and a semiconductor device, which contains the porous film inside.
2. Description of the Related Art
In the fabrication of semiconductor integrated circuits, as the circuits are packed tighter, an increase in interconnection capacitance, which is a parasitic capacitance between metal interconnections, leads to an increase in interconnection delay time, thereby hindering the enhancement of the performance of semiconductor circuits. The interconnection delay time is called an RC delay, which is in proportion to the product of the electric resistance of the metal interconnections and the static capacitance between the interconnections. Reducing the interconnection delay time requires reducing the resistance of metal interconnections or the interconnection capacitance.
The reduction in resistance of the interconnection metal and the interconnection capacitance can prevent a densely packed semiconductor device from causing an interconnection delay, thereby realizing a smaller and faster semiconductor device with reduced power consumption.
In an attempt to reduce the resistance of metal interconnections, in recent years, metallic copper interconnections have been employed more than conventional aluminum interconnections in the structure of a device. However, use of this structure all has limits in the enhancement of the performance, so the reduction in interconnection capacitance is an urgent necessity for higher performance of semiconductors.
One method for reducing interconnection capacitance is to reduce the relative permittivity (dielectric constant) of an interlevel insulator film disposed between metal interconnections. As such an insulator film with a low relative permittivity, it has been considered to use a porous film instead of a silicon oxide film, which has been used conventionally. A porous film can be said to be the only practical film as a material with a relative permittivity of 2.0 or less, and various methods for forming a porous film have been proposed
A first method for forming a porous film is as follows: a precursor solution of a siloxane polymer containing a thermally unstable organic component is synthesized; then the precursor solution is applied on the substrate to form a coating film; and later, a heat treatment is applied to decompose and volatilize the organic component. The result is a number of micro-pores formed in the film.
As a second method for forming a porous film, it is well known to carry out processing as follows: a silica sol solution is applied onto a substrate by coating or using a CVD method so as to form a wet gel; and then the silica sol is subjected to a condensation reaction while restricting volume reduction by controlling the speed of the evaporation of the solvent from the wet gel.
As a third method for forming a porous film, it is well known that a silica micro-particle solution is applied on a substrate to form a coating film, and then the coating film is sintered to form a number of micro-pores between silica micro-particles.
As a fourth method, Japanese Patent Provisional Publication No. 2000-44875 proposes a composition for porous film formation, which is characterized by containing a compound having (A) a component expressed by (R′)mSi(O R″)4-m (R′ and R″ are univalent organic radicals, and m is an integer of 0 to 2); (B) a metal chelate compound; and (C) a compound having a polyalkylene oxide structure.
However, these methods have respective major drawbacks as follows.
In the first method for forming a porous film, the synthesis of the precursor solution of the siloxane polymer increases the cost. In addition, the formation of the coating film by coating the precursor solution increases the amount of silanol groups remaining in the coating film, which causes a degassing phenomenon indicating the evaporation of water and the like in the heat treatment process that is conducted later and, which also deteriorates the film quality due to the porous film absorbing humidity.
In the second method for forming a porous film, the speed control of the evaporation of the solvent from the wet gel requires a special type of coating device, which increases the cost. In addition, a significant amount of silanol remains on the surface of the micro-pores, which must be silanized because otherwise hygroscopicity is high and the film quality decreases. The silanization makes the process more complicated. In the case where a wet gel is formed by the CVD process, it is necessary to use a special type of CVD device, which is different from the plasma CVD device generally used in the semiconductor process, thereby also increasing the cost.
In the third method for forming a porous film, the diameter of the micro-pores formed between the silica micro-particles, which is determined by the accumulation structure of the silica micro-particles that are accumulated geometrically, becomes very large. This makes it difficult to set the relative permittivity of the porous film to 2 or below.
In the case of the fourth method, out of the three components (A), (B), and (C), the metal chelate compound of (B) is essential to increase the compatibility of the components (A) and (C), and to make the thickness of the coating film uniform after being hardened. However, it is not preferable because it makes the manufacturing process complicated and increases the cost. Therefore, it is desired to develop a material, which enables a homogeneous solution to be formed without a chelate component and the coating film to be flat after being hardened.
Contrary to the conventional method for forming porous film, it has been found that aluminosilicate, silica and the like are first condensed using a mold of micelle formed by a surfactant and then the surfactant is removed from the produced construction by calcination or solvent extraction, which results in the formation of a porous structure having a mesopore (2–50 nm in diameter)—sized channel structure. For instance, according to Inagaki et al. (J. Chem. Soc. Chem. Commun., p. 680, 1993), polysilicate is reacted water in the presence of surfactant as a mold. Moreover, according to the Japanese Patent Provisional Publication No. 9-194298, the tetraalkoxysilane is reacted in water under an acidic condition in the presence of surfactant as a mold and coated on a substrate to form a silica porous film containing pores having pore diameter of 1 to 2 nm.
However, there is a problem with these methods. In the former, although a powdery porous body is easily made, a porous film cannot be formed on a substrate which is used for producing a semiconductor device. In the latter, although a porous film can be formed, the orientation of pores cannot be controlled so that uniform film in a large area thereof cannot be obtained.
Further, according to Japanese Patent Provisional Publication No. 2001-130911, the silica meso-porous film is formed using a mixture of a surfactant and a product obtained by acid hydrolysis and condensation of silicon alkoxide, where the mixture has been prepared and stabilized at pH value of 3 or less.
However, in this case, because a solute concentration is specified, it is difficult to control the coating film thickness. Then, it is difficult to apply this method to an actual semiconductor manufacturing process. Moreover, when this solution is diluted with water, although the thickness of the coated film can be controlled, the rate of condensation polymerization of the silica component increases, so that the stability of the coating liquid is lost.
On the other hand, the following methods are in the 2001-110529th according to the Japanese Patent Provisional Publication Nos. 2001-115029 and 2001-203197, a coating liquid is obtained by hydrolysis and condensation of a silane compound and provides excellent dielectric properties. Because a modulus of elasticity required in the manufacturing process of an actual semiconductor is 5 GPa or higher, however, the mechanical strength is not sufficient in these inventions.
As mentioned above, when porous film produced by conventional material is used as an insulator film in multi-level interconnects of the semiconductor device, there are problems such that moisture-absorbed porous film increases dielectric constant and lowers the adhesiveness in the lamination of films. Moreover, when the dielectric constant of the porous film used as an insulator film increases, the RC delay in the multi-level interconnects of the semiconductor device is increased. Consequently, the performance of the semiconductor device (high speed and low power consumption) has not been improved. Furthermore, higher cost for forming porous film leads to higher cost for a semiconductor device. Moreover, when conventional porous film is incorporated as an insulator film into the multi-level interconnects of the semiconductor device, there is a problem of the film having insufficient mechanical strength. The low mechanical strength results from the porous film property. Thus, there is a problem that low mechanical strength of the porous film as an insulator film lowers the reliability of a semiconductor device.